Tuba noise from engine compartment

Have a weird issue that has been going on for over a year. My car makes a sound like a tuba randomly. I never know when it is going to happen it just does. Sometimes I can get it to stop by revving the engine and sometimes I have to shut the car off to make it stop. It happens when it is cold out, warm out, in park , in drive, first thing in the morning or in the afternoon. Sometimes it does not happen for weeks then starts up again. I have tried cleaning the IAC and it still happens. Help!